The trackball and the mouse have been recognized as equivalent in many respects, such as in the introductory portion of U.S. Pat. No. 4,917,516 to Retter, which reads at column 5, lines 57 through 65: "Trackballs are essentially `upside down` mice. The major advantage over mice is that the location remains constant and close to the keyboard which allows easier alternation between the keyboard and the trackball. Trackballs require less desk space then most mice. The major disadvantage of a trackball, however, is that it is less controllable than a mouse and it is somewhat less intuitive in use. Like mice, however, trackballs require the hand to leave the keyboard,"
U.S. Pat. No. 4,595,070 to Hodges has the following language equating somewhat the mouse and the trackball at column 2, lines 30 though 35: "A mechanical mouse contains a track-ball as the movement sensing element. Such a ball is mounted in a box along with several position encoders. When the ball rolls across a surface, its rotation speed is approximately proportional to the speed at which a cursor moves upon a CRT screen."
U.S. Pat. No. 4,562,347 to Hovey is directed to a combined mouse and trackball. This is achieved by having the control ball protrude from both sides of the device, which differs basically from the instant invention. Moreover, when the device of the Hovey patent is used as a trackball, it can not be simply rested on a surface since the control ball would not be free to move by hand manipulation. Also, when the operation is that of a mouse, the operator must avoid interference with the control ball from the top as the operator grasps and moves the device.
A prior combination mouse and trackball sold under the name Penny+Giles also has the control ball protrude from both sides of the device. The device has an upward slope when positioned as a trackball with the ball near the front end. The upward slope positions the bottom of the ball above a surface on which the device rests. The device is substantially longer than a standard mouse. When used as a mouse, the device is tipped forward by its operator to bring the front end into contact with the surface on which the device rests. The bottom surface terminates behind the trackball, and the front and back contacts are located to bring the ball in contact with surface to be moved frictionally by the surface when moved as a mouse.
Accordingly, when the device is operated as a mouse, the operator must continuously tip it forward, which is subject to error and fatigue.